This invention relates to non-linear integration circuits, and more particularly to an improvement of a conventional non-linear integration circuit.
FIG. 1 shows a conventional non-linear integration circuit which is generally used. An audio signal is applied to an input terminal 1 and is then amplified by an amplifier 2. The signal thus amplified is applied through a resistor 3 and a half-wave rectifier circuit 4 to a smoothing circuit 5 with a diode 6, as a result of which a DC output is provided according to the amplitude of the input signal and is supplied to the impedance element R of a Field-Effect Type transistor, etc. FIG. 2 shows waveforms at each point of the above-described circuit. More specifically, part (I) of FIG. 2 shows the waveform of the amplified sine wave, part (II) shows the output waveform of the halfwave rectifier circuit 4 which includes ripples, and part (III) illustrates the output of the smoothing circuit 5 with the diode 6, which is a substantially direct current.
The function of the diode 6 included in the non-linear integration circuit in FIG. 1 will be described.
It is assumed that the diode 6 is eliminated from the integration circuit in FIG. 1 and the amplitude of the signal is abruptly increased as indicated at (A) in the part (I) of FIG. 2. In this case, the charging of a smoothing capacitor 8 is delayed by the time constant which is determined by a resistor 9 and the capacitor 8, as a result of which an output waveform as indicated by the dotted line (C) in part (III) of FIG. 2 is obtained. That is, if the integration circuit is not provided with the diode 6, then the circuit cannot respond to the abrupt amplitude change. On the other hand, in the case where the circuit includes the diode 6, when a signal whose amplitude exceeds the threshold level of the diode 6 is inputted, the current for charging the capacitor 8 flows through the diode 6 instead of the resistor 9. Therefore, a DC output corresponding to the input signal amplitude is obtained as indicated by the solid line (B) in part (III) of FIG. 2 without being delayed as indicated by the dotted line (C).
Thus, when the input signal amplitude is abruptly increased, the charging current flows quickly into the smoothing capacitor 8 through the diode 6; that is, the integration circuit can respond to the change of the input signal amplitude.
However, the above-described circuit still suffers from a problem if the diode 6 is a silicon diode. The threshold voltage of the silicon diode is about 0.7 volt. The silicon diode may be used in the case when the supply voltage is relatively high, but it cannot be used when it is relatively low, 1.8 to 3 volts. For instance when the supply voltage is 1.8 volts and the signal is superposed on a DC voltage of 0.9 volts, then the effective supply voltage is only 1.1 volts because the diode's voltage loss is 0.7 volt. In addition, the range of amplitude about the DC voltage (0.9 V) is only .+-.0.2. Accordingly, the silicon diode cannot be used when the supply voltage is low.
Instead of the above-described silicon diode, a germanium diode may be used and be operated on low voltage; however, it is still disadvantageous in that the use of the germanium diode obstructs the provision of the whole circuit in the form of an integrated circuit and the manufacturing cost is increased.
Furthermore, it is necessary that the non-linear integration circuit according to the invention is generally interchangeable with other such circuits in a sound recording and reproducing device. Accordingly, it is also required that the non-linear integration circuit according to the invention be similar in characteristic to that which employs a silicon diode which is generally used.